Technical Field
The present invention relates to a carrier for forming images, and a developer including the carrier.
Description of the Related Art
Electrophotographic image forming methods include forming an electrostatic latent image on an image bearer such as a photoconductive material, transferring a charged toner thereto to form a visible image (toner image), transferring the toner image onto a recording medium such as paper, and fixing the toner image thereon to form a final output image. Recently, electrophotographic copiers and printers are rapidly developing from monochrome to full-color, and full-color markets are expanding.
Full-color image forming apparatuses are becoming oilless for the purpose of being downsized and simplified as well as monochrome image forming apparatuses. However, as mentioned above, to improve color reproducibility of a color toner, the color toner needs to have lower viscoelasticity because the fixed color toner image is required to have a smooth surface. Therefore, the color toner has offset problems more often than the monochrome toner does, making it more difficult to make a fixer oilless or use only a small amount of oil. In addition, a toner including a release agent has higher adherence to an image bearer and lower transferability to a transfer paper. Further, the release agent therein contaminates friction-charged members such as a carrier and lowers the chargeability thereof, resulting in deterioration of durability of the toner.
On the other hand, for the purpose of preventing toner constituents from filming, making the surface thereof uniform, preventing oxidization thereof, preventing deterioration of moisture sensitivity thereof, extending lives of developers, preventing adherence of the carriers to the surfaces of photoconductors, protecting photoconductors from being damaged or abraded by the carriers, controlling charge polarity thereof and controlling charge quantity thereof, a resin including carbon black is applied on the carrier core material to form a coating layer thereon. However, although quality images are produced initially, image quality may deteriorate because the coating layer is abraded as the number of images produced increases. Further, when the coating layer is abraded or the carbon black releases therefrom, color stains may occur. Instead of the carbon black, titanium oxide or zinc oxide can be used, but does not decrease volume resistivity sufficiently.
A toner tends to have low-temperature fixability to decrease power consumption. Further, printing speed is constantly increasing, and therefore a phenomenon known as toner spent on carrier more easily occurs. In addition, a toner tends to include many additives to produce images having higher quality, which increase toner spent, resulting in lowering the toner charge, toner scattering, and background fouling.
An expanding commercial printing market, i.e., a field of production printing needs higher quality images. It is quite difficult for a machine only to technologically suppress variation or uneven image density in an image and image density variation among ten thousands of images produced. Therefore, a toner is more required to have a constant charge quantity.